So-called two-component system heat-sensitive recording materials utilizing the coloration reaction between a colorless or slightly colored electron-donating dye precursor and an electron-accepting compound are disclosed, for example, in Japanese patent publication Nos. 14039/70 and 4160/68, etc.
These two-component system heat-sensitive recording materials comprise a support having coated thereon a colorless or slightly colored electron-donating dye precursor and an electron-accepting compound as fine particles in such manner that these two types of thermally reactive compounds are separated from each other by a binder or the like, one or both of the compounds becomes molten upon heating the materials, and thus, contact between the compounds occurs to cause a coloration reaction.
Such two-component system heat-sensitive recording materials have the advantages that: (1) they utilize a primary coloration reaction and, therefore, do not require development; (2) they possess paper quality which approximates that of ordinary paper; (3) they can be handled with ease; (4) they provide high density coloration; and (5) heat-sensitive recording materials providing various color hues can be easily prepared. Thus, these materials have tremendous advantages and, hence, are most often utilized as heat-sensitive recording materials.
In recent years, this type of heat-sensitive recording materials have been particularly utilized in the field of facsimiles, recorders, and printers. With recent increasing usage in the field of information transmission via facsimiles, recording speed has been concomitantly increased. As to heat-sensitive recording materials, color-forming properties inherent in such materials which are capable of forming color with sufficient density by a short pulse width, or a low heat energy (i.e., an improved heat responsibility) have been strongly desired, and various attempts along these lines have been made, as disclosed, for example, in Japanese patent application (OPI) Nos. 187393/83, 67083/84, 54884/85, etc. (the term "OPI" as used herein refers to a published unexamined Japanese patent application"). However, increased heat responsibility tends to increase fog density in the white background portion of the recording materials; thus, commercial value of these recording materials is often undesirably lowered. Therefore, various attempts have been made, such as disclosed in Japanese patent application (OPI) Nos. 169889/84, 205796/83, 167297/84, etc., to solve this problem of fog density in the white background portion. However, all of these attempts have the specific defect in that fog in the white background area is not sufficiently prevented, or that heat responsibility is seriously reduced when fog formation is prevented to some extent.